As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having a high energy density and a high discharge voltage, on which much research has been carried out and which is now commercialized and widely used.
When the secondary battery is used as a power source for mobile phones or laptop computers, it is required for the secondary battery to stably provide a predetermined level of power. On the other hand, when the secondary battery is used as a power source for power tools, such as electric drills, it is required for the secondary battery to instantaneously provide a high level of power and, at the same time, be stable against external physical impacts, such as vibration or dropping.
FIG. 1 is a vertical sectional view illustrating the structure of a conventional cylindrical secondary battery.
Referring to FIG. 1, the cylindrical secondary battery 10 generally includes a cylindrical container 20, a jelly-roll type electrode assembly 30 mounted in the container 20, a cap assembly 40 coupled to the upper end of the container 20, and a crimp region 50 at which the cap assembly 40 is mounted.
The electrode assembly 30 is constructed in a structure in which cathodes 31 and anodes 32 are wound in a jelly-roll shape while separators 33 are respectively interposed between the cathodes 31 and the anodes 32. To the cathodes 31 is attached a cathode tab 34, which is connected to the cap assembly 40. To the anodes 32 is attached an anode tab (not shown), which is connected to the lower end of the container 20.
The cap assembly 40 includes a top cap 41 constituting a cathode terminal, a positive temperature coefficient (PTC) element 42 for intercepting electric current through the great increase of battery resistance when the interior temperature of the battery increases, a bent safety member 43 for intercepting electric current and/or discharge gas when the interior pressure of the battery increases, an insulating member 44 for electrically isolating the bent safety member 43 from a cap plate 45 excluding a specific portion, and the cap plate 45 connected to the cathode tab 34, which is attached to the cathode 31. The cap assembly 40 is constructed in a structure in which the top cap 41, the PTC element 42, the bent safety member 43, the insulating member 44, and the cap plate 45 are sequentially stacked.
The crimp region 50 is formed at the upper end of the container 20 such that the cap assembly 40 can be mounted to the open upper end of the container 20. More specifically, the crimp region 50 is formed by beading the upper end of the container 20, such that a depression 21 is formed at the inside of the container 20, mounting a gasket 60, sequentially inserting the outer circumferential parts of the cap plate 45, the insulating member 44, the bent safety member 43, and the top cap 41, and bending the upper end of the container 20. As a result, the crimp region 50 is formed in the shape to surround the gasket 60 located at the inside of the crimp region 50. The cap assembly 40 is mounted at the crimp region 50 by crimping and pressing.
However, it has been proven that, when external impacts are applied to the cylindrical secondary battery with the above-stated construction, the sealability of the cylindrical secondary battery is lowered, the resistance at the electrical connection regions of the cylindrical secondary battery is changeable, and the safety of the cylindrical secondary battery is lowered, whereby it is difficult for the cylindrical secondary battery to exhibit desired battery performance.
For this reason, the inventors of the present invention proposed a secondary battery having an improved structure as shown in FIG. 2, which is disclosed in Korean Patent Application No. 2006-22950.
Referring to FIG. 2, the cylindrical secondary battery 100 is manufactured by inserting an electrode assembly 110 into a container 200, injecting an electrolyte into the container 200, and mounting a cap assembly 300 to the upper end, which is open, of the container 200. The process for manufacturing the cylindrical secondary battery 100 is generally identical to the process for manufacturing the conventional cylindrical secondary battery. However, the structure of the cylindrical secondary battery 100 is different from that of the conventional cylindrical secondary battery. The difference will be described below in detail.
The cap assembly 300 is mounted to the open upper end of the container 200 by a crimp region 500 constructed in a structure in which a top cap 310 and a bent safety member 320 for lowering the interior pressure of the battery are in tight contact with each other inside a gasket 400 mounted to an upper beading part 210 of the container 200 for maintaining airtightness. The top cap 310 is formed such that the central part of the top cap 310 protrudes upward, and therefore, the top cap 310 serves as a cathode terminal, to which an external circuit is connected. The top cap 310 is provided along the circumference of the protruding part thereof with a plurality of through-holes 312, through which pressurized gas is discharged out of the container 200.
The bent safety member 320 is a thin-film structure through which electric current flows. The central part of the bent safety member 320 is depressed to form a depressed central part 322, and two notches 324 and 326 having different depths are formed at upper and lower bent regions of the central part 322, respectively. Below the bent safety member 320 is mounted a current intercepting member 600 for discharging gas out of the battery and, at the same time, intercepting electric current.
The end 328 of the bent safety member 320 surrounds the outer circumference 314 of the top cap 310, and an annular protrusion 316 is formed at the lower end surface of the top cap 310.
The cylindrical secondary battery 100 with the above-stated construction solves the sealability-related problem and the resistance change problem at the electrical connection regions. However, the experiments carried out by the inventors of the present invention revealed that, when external impacts are continuously applied to the cylindrical secondary battery, the crimp region constructed in the structure of FIG. 1 as well as the crimp region constructed in the structure of FIG. 2 is easily deformed, with the result that the contact surfaces between the top cap, the bent safety member, and the gasket are separated from each other, whereby the sealability of the cylindrical secondary battery is lowered.
FIG. 3 is a partially enlarged view illustrating the crimp region of the cylindrical secondary battery shown in FIG. 2. For convenience of description, only the section of the container forming the crimp region is illustrated.
Referring to FIG. 3, the crimp region 500 is constructed in a structure in which the end of the crimp region 500 is bent such that the cap assembly 300 (see FIG. 2) is stably mounted to the open upper end of the container 200 (see FIG. 2) while the gasket 400 (see FIG. 2) is disposed between the cap assembly 300 and the open upper end of the container 200. A bent front end 510 of the crimp region 500 extends inward while the bent front end 510 of the crimp region 500 is inclined in a predetermined angle such that the bent front end 510 of the crimp region 500 presses the gasket 400 (see FIG. 2) to provide a high sealability. For most cylindrical secondary batteries, the bent region has a radius of curvature (R) of approximately 1.3 mm or more.
In this structure, however, when an external force is applied frequently to the side of the battery (in the direction indicated by a horizontal arrow), the crimp region 500 is deformed in the shape of a dotted line, with the result that the sealed state of the gasket is partially released, and therefore, the contact surfaces between the bent safety member and the top cap are instantaneously spaced apart from each other. As a result, an electrolyte leaks from the cylindrical secondary battery through the gap defined between the bent safety member and the top cap, whereby the safety of the cylindrical secondary battery is greatly lowered. Also, when the internal pressure of the cylindrical secondary battery increases, the above-mentioned deformation occurs due to the pressure applied from the inside of the cylindrical secondary battery, with the result that the electrolyte leaks from the cylindrical secondary battery.
It may be considered to decrease the radius of curvature, when a bending process for crimping is carried out, in order to restrain the deformation of the crimp region due to the external impacts applied to the cylindrical secondary battery and the increase of the internal pressure of the cylindrical secondary battery. In this case, however, the bent region does not slop gently due to the small radius of curvature, with the result that wrinkles may formed at the container.
Also, it may be considered to bend the front end of the crimp region, such that the bent front end of the crimp region presses approximately perpendicularly against the gasket, in order to greatly increase a force applied to the gasket in the cylindrical battery. An example of the structure is partially disclosed in drawings of U.S. Pat. No. 5,150,602 and No. 4,656,736. When the bent front end of the crimp region presses perpendicularly against the gasket, however, a fatigue phenomenon of the elastic material constituting the gasket greatly increases, whereby cracks may occur due to the external impacts and the increase of the internal pressure, and therefore, the sealability of the cylindrical battery is sharply reduced.
Meanwhile, the outer surface of the container of the cylindrical secondary battery is covered generally by an insulative tube to insulate the outer surface of the container, excluding electrode terminal regions, from the outside and to prevent the outer surface of the container from being damaged by scratches.
The insulative tube has been normally made of poly vinyl chloride (PVC). However, the PVC tube has a low heat resistance, secondary contraction occurs in the PVC tube during the high-temperature treatment of the PVC tube, and noxious substances are generated from the PVC tube when the PVC tube is discarded, with the result that several problems, such as environmental pollution, occurs from the PVC tube. For this reason, the insulative tube is mainly made of other polymer resins instead of the PVC.
However, the experiments carried out by the inventors of the present invention revealed that such a tube has a low impact resistance, and therefore, when an external force is applied to a secondary battery used as a power source for power tools, the tube did not exhibit a desired level of impact absorptivity. Therefore, there is a high necessity for a cylindrical secondary battery exhibiting more excellent characteristics by the improvement of the crimping structure and the insulative tube.